Arabic

ṯuknat- - pearl necklace; dyed wool collar that hangs as an ornament on the camel's neck (BK 1 231)

presumably related with assimilation of *m > n after the velar. As for the semantic shift, see SED I, No. 15

PWS

*ṯVkm- - the lowest part of the neck (SED I No. 281; Kogan 2011: 216)